The present invention relates to ceramic substrates for microelectronic circuits and to a process for producing the substrates. More particularly, the invention is directed to a ceramic substrate having a specially low dielectric constant, low coefficient of thermal expansion and high mechanical strength and permitting wiring thereon with a high-melting metallic conductor, and to a process for producing such ceramic substrates.
In recent years, with the increasing integration degree of semiconductor devices, there are growing needs for circuit substrates supporting such devices to accept higher-density wiring and to have higher performance characteristics, higher reliability, and so forth. In particular, subjects important to circuit substrates for use in electronic computers and the like are high-speed signal propagation and high reliability. For these substrates, there are used, in practice, ceramics composed mainly of alumina (Al.sub.2 O.sub.3).
Desired characteristics of the ceramic to be used for such circuit substrates are generally as follows:
(1) The ceramic insulator is dense and has a hermetic nature. This matter relates to the overall reliability of the circuit substrate.
(2) The coefficient of thermal expansion of the ceramic is as close as possible to that of silicon chips. This is for the purpose of minimizing strain which will develop at the junction between the ceramic substrate and the silicon chip to prolong the joint life and enhance the reliability.
(3) The dielectric constant of the ceramic is minimized. This is for speed-up of the signal propagation.
(4) Junction of conductor metals to the ceramic substrate is strong, that is, the metallized bond strength is high. This relates to the bond strength between the circuit substrate and the output or input terminal.
(5) The ceramic has a high mechanical strength. This is necessary for handling in the process for fabricating the substrate and for mounting onto a sealing means and a cooling means to the substrate.
Thus the material to be used for the circuit substrates should satisfy the above requirements simultaneously. In particular, circuit substrates each loaded with several tens densely integrated semiconductor components for use in electronic computers will be inapplicable practically if any one of the above items is not satisfied.
Conventionally Al.sub.2 O.sub.3 is used for substrates of this type. Although it is satisfactory in hermetic nature, metallized bond strength and mechanical strength, it has a higher coefficient of thermal expansion of 8.times.10.sup.-6 /.degree.C. than that of silicon chips (3.times.10.sup.-6 /.degree.C.) and also has a high dielectric constant of about 10. Accordingly, Al.sub.2 O.sub.3 is not suitable for circuit substrates.
Known ceramic insulators having a lower coefficient of thermal expansion and dielectric constant than that of Al.sub.2 O.sub.3 include silica (SiO.sub.2, .epsilon.=ca. 4), cordierite crystal (5SiO.sub.2.2Al.sub.2 O.sub.3.2MgO, .epsilon.=ca. 5.0), cordierite glass (.epsilon.=6.3), steatite (MgO.SiO.sub.2, .epsilon.=6.3), forsterite (2MgO.SiO.sub.2, .epsilon.=6.5), and mullite (3Al.sub.2 O.sub.3.2SiO.sub.2, .epsilon.=7).
However, the coefficient of thermal expansion of SiO.sub.2 and cordierite crystal are very low, i.e., as low as 5.times.10.sup.-7 /.degree.C. and 1.5.times.10.sup.-6 /.degree.C., respectively, and those of steatite and forsterite are 7.2 and 9.8 (room temperature - 400.degree. C.), respectively, which are nearly equal and higher than that of Al.sub.2 O.sub.3. The coefficient of thermal expansion of cordierite glass is about 3.7.times.10.sup.-6 /.degree.C., which is close to that of silicon chips, but the mechanical strength of cordierite glass is as low as 100 MPa, so that the cordierite glass is impractical for circuit substrate purposes.
Mullite is somewhat unsatisfactory in dielectric constant and coefficient of thermal expansion, but it has a high mechanical strength of 350 MPa, which is thus most promising among the conventional ceramics.
However, mullite has the following inherent problems (1) and (2):
(1) Bond strength between mullite and a usual conductor metal is markedly low. This is because no chemical reaction occurs between mullite and either tungsten (W) or molybdenum (Mo), which is used commonly as a conductor metal on alumina substrates and the like, even at elevated temperatures. This property is inherent in mullite.
(2) Highly strengthening of the above-mentioned bond requires a special powder of mullite and a special sintering method which are impractical as well as expensive. That is, K. S. Mazdiyasni and L. M. Brown ["Synthesis and Mechanical Properties of Stoichiometric Aluminum Silicate (Mullite)", J. Am. Ceramic Soc., 55[11], 548-555 (1972)] obtained a sintered mullite body capable of forming a high strength by compacting a fine powder of mullite and sintering the compacted body at a temperature as high as 1800.degree. C.
It is very difficult, however, to form such a powder in green sheets (before sintering), which are preforms of circuit substrates. Moreover, the sintering temperature of 1800.degree. C. is much higher than those used for usual substrates, e.g., 1500.degree. to 1650.degree. C. This is a significant bottleneck in practicing this method in view of also the heating elements and heat insulator of the furnace.
While mullite is inherently hard to sinter, as described above, there has long been used a method referred to as "liquid phase sintering" which has been reduced into practice for producing sintered hard alloys.
The typical sintered hard alloy is composed of tungsten carbide (WC) and cobalt (Co). Although WC is difficult to sinter in single form, it can be made into a high-density sintered body when burned jointly with several percentages of cobalt. This is because cobalt is melted in the sintering step and the melted cobalt draws WC in the solid phase thereto by the surface tension thereof.
This liquid phase sintering method is also applied to the sintering of Al.sub.2 O.sub.3 for producing circuit substrates therefrom. That is, usual Al.sub.2 O.sub.3 particles of several .mu.m in size are hard to sinter, but they can be densely sintered according to the liquid phase sintering mechanism by addition of a material (an eutectic composition of three or four components such as SiO.sub.2, Al.sub.2 O.sub.3, MgO, and CaO) fusible at a far lower temperature than is Al.sub.2 O.sub.3.
In the above two examples, both cobalt and the three- or four-component eutectic composition, which generate a liquid phase, play the role of promoting the sintering of a hardly sinterable substance. Nevertheless, the former is called a binder and the latter a sintering aid, in general.
The reason for the above is as follows: In the case of the WC-Co sintered hard alloy, WC crystal grains are strongly bonded together through metallic cobalt, and the high hardness and high toughness of this alloy can be altered optionally with the combination of hard and brittle WC and tough cobalt. Thus the binder function of cobalt is very effective.
In the case of the Al.sub.2 O.sub.3 circuit substrate, the sintering of Al.sub.2 O.sub.3 can be greatly promoted by addition of the three- or four-component eutectic composition, but the original properties of Al.sub.2 O.sub.3 are scarcely varied by this addition. Therefore, the three- or four-component eutectic composition is generally called a sintering aid.
From the above described point of view, studies of sintering aids for mullite have been made for the purpose of solving difficulties in sintering mullite ceramics. Of course, these studies are all intended to make denser the texture of mullite according to the liquid phase sintering mechanism by using cordierite as another sintering aid.
For instance, in Japanese Patent Laid-Open No. 139709/80 and in "Preparation and Properties of Mullite-Cordierite Composites" [B. H. Mussler and M. W. Shafer, Am. Ceram. Soc. Bull., 63, 705 (1984)], discussion is given on the use of mullite as a matrix and cordierite as a sintering aid.
From the equilibrium diagram of the SiO.sub.2 -Al.sub.2 O.sub.3 -MgO system, it can be seen that the melting point of 5SiO.sub.2.2Al.sub.2 O.sub.3.2MgO is 1490.degree. C., which is far lower than the melting point (1830.degree. C.) of mullite. Thus the mullite texture has been made denser by the liquid phase sintering action, yielding a sintered body of zero % water absorption.
While the sintering aid used in Japanese Patent Laid-Open No. 139709/80 and the B. H. Mussler et al article are equally referred to as cordierite, it is not clear from the former whether the cordierite is crystalline or amorphous, and B. H. Mussler et al use crystalline cordierite.
Cordierite either in a crystalline or amorphous form has a lower coefficient of thermal expansion and a lower dielectric constant than those of mullite as stated above. Accordingly, it is expected that the addition of cordierite to mullite will lower the coefficient of thermal expansion and dielectric constant of mullite as well as produce the sintering promoting effect
In the Laid-Open No. 139709/80, a sintered body having a coefficient of thermal expansion ranging from 4.2.times.10.sup.-6 to 3.8.times.10.sup.-6 /.degree.C. and dielectric constant ranging from 6.7 to 6.5 is obtained when the proportion of cordierite to mullite is altered from 3.63 to 36.2% by weight.
In the Laid-Open No. 139709/80, while cordierite is incorporated into a mullite crystal matrix, the composition range within which the above-mentioned characteristics are obtained is expressed in terms of MgO, Al.sub.2 O.sub.3 +SiO.sub.2, and the weight ratio of Al.sub.2 O.sub.3 /SiO.sub.2. Such expression of composition is obviously inappropriate for sintered bodies made denser by the liquid phase sintering mechanism and for sintered bodies all the characteristics of which are dependent on Al.sub.2 O.sub.3 crystal matrix. It is reasonable to express the compositions of sintered mullite-cordierite bodies in terms of the proportion of cordierite to mullite.
According to the article of B. H. Mussler et al., sintered bodies having a coefficient of thermal expansion ranging from 4.5.times.10.sup.-6 to 3.2.times.10.sup.-6 /.degree.C. and dielectric constant ranging from 5.7 to 4.8 are obtained when the proportion of crystalline cordierite to mullite is altered from 17.1 to 76.8% by weight.
In the two prior art examples described above, the obtained sintered bodies, when used for circuit substrates, are nearly satisfactory in air tightness, coefficient of thermal expansion and dielectric constant.
The mechanical strength of ceramics, that is, one of the characteristics required for circuit substrates is not described in the two prior art examples. Hence, it is doubtful whether these prior art ceramics are satisfactory in strength when used as circuit substrates.
Moreover, no result of investigation on metallized bond strength is described in the prior art examples. Simultaneous aggregative sintering of a conductor metal with an insulator ceramics is indispensable particularly for fabricating multilayer circuits comprising a number of substrates. Nevertheless, no description is given on the metallized bond strength in the prior art examples. It is a fatal matter in using these ceramics for circuit substrates if the metallized bonds thereof are weak.
The reason for giving no result about the metallized bond strength in the prior art examples may be that the sintering aids used in the examples have fundamental defects which affect the metallizing of mullite substrates.
Since mullite does not react chemically with any of such high-melting metals as W and Mo, the liquid phase penetration method that is applied to Al.sub.2 O.sub.3 substrates and the like is indispensable in order to join firmly such metals with mullite.